This question follows from this other question where the OP mentions that:

I know MOVs inside surge protectors degrade over time

Based on this it seems that surge protectors eventually deteriorate past a point where they do not provide (expected) protection against power surges.

The questions I wanted to ask about this:

  1. Is it correct that surge protectors degrade, and if it is, what causes this?
  2. Is there a rule of thumb for determining how long a surge protector can last before needing to be replaced?
  3. Are there ways to determine if a surge protector is old (or otherwise faulty, for that matter)?
  • \$\begingroup\$ Surge protectors don't provide much protection. You need a UPS or some other equipment to provide isolation. \$\endgroup\$
    – Voltage Spike
    Commented Aug 15, 2016 at 20:37
  • \$\begingroup\$ If your asking about MOV's degrading over time 1)Have you done any research on this before posting? The other questions are off-topic. \$\endgroup\$
    – Voltage Spike
    Commented Aug 15, 2016 at 20:40

3 Answers 3


It truly depends whether your surge protector contains MOVs or not.

The wear mechanism for a MOV is that the clamping voltage DECREASES slightly every time the MOV is required to clamp a transient. Eventually, the clamping voltage decreases to the point where the MOV starts to conduct on the peaks of every AC cycle. The MOV will then burn up.

FWIW - the early cube plug-in surge protectors manufactured by RCA had a clear plastic window on the side that showed the red MOV inside. A label on one side of the cube said something along the lines of; "If it's black, take it back."

More modern surge protector power bars have thermal cutouts mounted between the MOVs. If a MOV flames, the thermal cutout interrupts the incoming power.

  • \$\begingroup\$ Surge protectors have a rating, usually a year. High end surge protectors will normally give some kind of monetary protection for the connected equipment if the protector is replaced according to the manufacturers rating. \$\endgroup\$
    – vini_i
    Commented Oct 5, 2015 at 10:42
  • \$\begingroup\$ So how would you know if the surge protector contains MOVs...would this be mentioned in the manual? Should surge protectors just be replaced every X months/years? \$\endgroup\$
    – user64167
    Commented Oct 11, 2015 at 16:48

Repeated surges will degrade a MOV over time by reducing the clamping voltage like Dwayne Reid said. You could test an unknown MOV on a megger or anything that applies a high voltage at a low current. A normal DVM applies a very low voltage on its ohm function so it won't reliably tell you if the MOV is on the way out. It will of course tell you if the MOV is shorted.

I have seen fuses with neon lamps across them placed in series with an MOV. When the MOV blows, the fuse blows stopping a fire, and the neon lights up indicating that the MOV is gone.


Quoting this Leviton article:

  • MOV life is largely dictated by thermal stress
  • Exposure to surges cause heat-ups and cool-downs of the device, as well as (depending on the nature of the surge) incremental damages such as electrical puncture and thermal cracking
  • These incremental damages lead to non-uniform heating when additional stresses are applied, which leads to thermal runaway and device failure

A surge protector that is connected to the mains should be considered 'on the clock' - once you get past the manufacturer's recommended replacement time period, you're on your own. Unless you have access to a surge generator you won't really be able to test if the MOV is still 'good', and since you could be in a 'one-and-done' situation (the next surge gets clamped but takes the MOV out), it's not really worth testing it.

You could (theoretically) replace a questionable MOV with a comparably-rated one if you just want protection and don't care about dealing with manufacturer's warranties. They're available from a variety of suppliers - energy-handling and voltage clamping level are the key spec items to look out for.

The safest thing to assume is that if the surge protector is older than the manufacturer's equipment guarantee, assume the MOV isn't in there at all (i.e. there's no protection).


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